In recent years, the growth of the nursery and lawn care industries has created a demand for compact, inexpensive yet accurate fluid proportioning and mixing systems. Ideally, a system of this type should operate effectively to mix, in a preset proportion, a chemical with a stream of solvent, such as water, and the ratio of chemical to water must be accurately maintained for a normal range of variables which affect both the water and chemical supplies. The system should accommodate both a wide range of primary flows and also provide the ability to adjust readily and continuously the proportioning ratio over a range of continuously desirable settings. In addition, the proportioning and mixing unit must be compact and formed of material which is both inert to a wide range of chemical ingredients normally mixed with water and which remains operative over a wide range of temperatures. The device should be driven by power derived from the water supply without creating a substantial pressure drop in the water supplied as a solvent, and the water supply should be positively protected from backflow contamination by the chemical used as a solute.
Previous proportioning and mixing systems have been designed to meet and satisfy some of the requirements for an ideal system, but none to this point has successfully met all such requirements. For example, relatively effective but inexpensive Venturi type proportioning and mixing systems have been developed as illustrated by U.S. Pat. Nos. 4,174,812, 4,247,046 and 4,277,030 to V. Hechler. These devices are compact and portable, and maintain mixing ratios within acceptable ranges for many applications. However, Venturi units are essentially fixed flow devices and not readily adjustable for proportioning ratio. They are also often subject to ratio variations caused by a number of variables such as solute viscosity, pressure and temperature variations in the supplied solute and solvent, and nozzle deterioration caused by contamination and chemical action.
More expensive pump type proportioning and mixing systems have been designed which are more accurate than the Venturi mixers and proportioners, for these pump type units positively measure the chemical solute in a measuring chamber having a defined volumetric capacity. Most pump type units include an alternator driven by the input water for the device which controls the direction of movement of two pistons individually mounted in one of two similar measuring chambers. The alternator may consist of two parallel spool valves, each of which operates selectively in response to water pressure to either direct water into a measuring chamber behind a piston or to vent water from the measuring chamber to an output. The pistons are interconnected by a cord or other mechanical connection, and when one measuring chamber is receiving water through a spool valve, the second is venting water through the remaining spool valve to the output. As water is vented, the piston draws a measured volume of solute into the measuring chamber, while the water entering the remaining measuring chamber causes the piston therein to expel to a mixer solute previously measured. A reversing switch mechanically actuated in response to piston position causes the spool valves to reverse operation when each piston reaches the extent of its travel within a respective measuring chamber.
Most pump type proportioning and mixing systems employ all water received from an external supply source to operate the alternator and to drive the pistons in the measuring chambers. Water expelled from the measuring chambers is directed to a mixer where it is mixed with solute provided from the measuring chambers. This use of all supply water results in excessive pressure losses in the water stream before it reaches the mixer, for all of the water supplied is required to travel the distances between the operating components of the unit as well as to provide the power for these operating components.
Attempts to use only a small portion of the supply water to power the alternator and pistons of prior pump type proportioning and mixing systems have met with only limited success, for the power available from this limited water supply is minimal. It has been necessary for the limited supply of drive water to travel through elongated tubes and conduits with an attendant pressure loss to reach the spaced working components of prior units, and the power remaining was often barely enough to drive the unit under ideal conditions. A factor which was likely to render inadequate the available power provided by the limited supply of drive water was the variable lift height effect which determines the energy necessary to lift the solute from a storage container to a measuring chamber.
In known pump type proportioning and mixing systems, where drive water is diverted from a main water stream, pressure losses in the main water stream still occur as it travels across the unit to an input valve for a mixer. Also, in such systems, destruction of the coupling unit between the two pistons is a common occurrence. When the solute supply tank is empty or when the system first begins to pump solute, air is drawn into one of the measuring chambers instead of solute. When this piston reverses to expel solute, it is opposed only by air, but the remaining piston is opposed by a solid column of water, and the resulting overload on the coupling mechanism between the two pistons often results in the destruction of the coupling mechanism.
Finally, pump type proportioning and mixing devices are generally large units including numerous fluid conducting tubes and passages, and this multiplicity of components increases the likelihood that some chemical solute will attack and destroy one or more components of the unit.